In this citizen journalism photo provided by the Local Committee of Arbeen, which has been authenticated based on its contents and other AP reporting, a man and woman mourn over the dead bodies of Syrian men after an alleged poisonous gas attack fired by regime forces, according to activists in Arbeen town, Damascus, Syria, Wednesday, Aug. 21, 2013. Syrian regime forces fired intense artillery and rocket barrages Wednesday on the eastern suburbs of the capital Damascus, in what two pro-opposition groups claimed was a "poisonous gas" attack that killed dozens of people. (AP Photo/Local Committee of Arbeen)

UNITED NATIONS (AP) – U.N. inspectors said Monday there is “clear and convincing evidence” that chemical weapons were used on a relatively large scale in an attack last month in Syria that killed hundreds of people in the country’s civil war.

The findings represent the first official confirmation by scientific experts that chemical weapons were used in the Syrian conflict, but the first page of the report, seen by The Associated Press, left the key question of who launched the attack unanswered. The U.S. has said the regime of President Bashar Assad was behind the Aug. 21 attack, while the Syrian government accuses the rebels.

Secretary of State John Kerry briefed U.S. allies on a broad agreement reached over the weekend with Russia to end Syria’s chemical weapons program, pressing for broad support for the plan that averted U.S. military strikes. Kerry met in Paris with his counterparts from France, Britain, Turkey and Saudi Arabia before seeking a U.N. resolution that would detail how the international community can secure and destroy Syria’s stockpile and precursor chemicals.

And in Geneva, the chairman of a U.N. war crimes panel said it is investigating 14 suspected chemical attacks in Syria, dramatically escalating the stakes. Paulo Sergio Pinheiro said the panel had not pinpointed the chemical used or who is responsible.

Secretary-General Ban Ki-moon presented the U.N. inspectors’ report to a closed meeting of the U.N. Security Council on Monday morning.

The inspectors said “the environmental, chemical and medical samples we have collected provide clear and convincing evidence that surface-to-surface rockets containing the nerve agent sarin were used … in the Ghouta area of Damascus” on Aug. 21.

“The conclusion is that chemical weapons have been used in the ongoing conflict between the parties in the Syrian Arab Republic, also against civilians, including children, on a relatively large scale,” the inspectors said in their report to Ban.

“This result leaves us with the deepest concern,” the inspectors said.

The inspectors were mandated to report on whether chemical weapons were used and if so which ones – not on who was responsible.

The rebels and their Western and Arab supporters blame President Bashar Assad’s regime for the attack in the rebel-controlled area of Ghouta. The Assad regime insists that the attack was carried out by rebels. The U.N. report mentions the Ghouta areas of Ein Tarma, Moadamiyeh and Zamalka, all of which were featured in the videos of victims that emerged shortly after the attack.

The Aug. 21 chemical attack unfolded as the U.N. inspection team was in Syria to investigate earlier reported attacks. After days of delays, the inspectors were allowed access to victims, doctors and others in the Damascus suburbs.

Chief weapons inspector Ake Sellstrom handed over the report to Ban on Sunday. In the report, Sellstrom said the team was issuing the findings on the Ghouta attacks “without prejudice” to its continuing investigation and final report on the alleged use of chemical weapons in three other areas. The letter said it hoped to produce that report as soon as possible.

Under an Aug. 13 agreement between the U.N. and the Syrian government, Sellstrom’s team was scheduled to investigate an alleged chemical weapons attack on March 19 on the village of Khan al Assal outside Aleppo and alleged attacks on two other sites which were kept secret for security reasons.

The inspectors’ report for the first time identified the two sites still to be investigated as Sheik Maqsood and Saraqueb.

The report also thanked the four laboratories designated by the Office for the Prohibition of Chemical Weapons (OPCW) to examine the samples from Syria, disclosing their locations for the first time – in Finland, Germany, Sweden and Switzerland.

Kerry ended a weeklong diplomacy tour in Paris on Monday after laying out with his French and British counterparts a two-pronged approach in Syria. They called for enforceable U.N. benchmarks for eradicating the chemical weapons program and an international conference bolstering the moderate opposition.

Kerry was pressing for support for the ambitious agreement reached with the Russians. It calls for an inventory of Syria’s chemical weapons program within one week, with all components of the program out of the country or destroyed by mid-2014.

France and the U.S. insisted that a military response to the Aug. 21 attack remained on the table, and were pressing for a U.N. resolution reflecting that in coming days.

“It has to be strong, it has to be forceful, it has to be real, it has to be accountable, it has to be transparent, it has to be timely. All of those things are critical. And it has to be enforced,” Kerry said. “We will not tolerate avoidance or anything less than full compliance by the Assad regime.”

Kerry said the agreement “fully commits the United States and Russia to impose measures under Chapter 7 of the U.N. Charter in the event of non-compliance.” Chapter 7 resolutions allow for military enforcement.

Russia’s Lavrov said Chapter 7 was the subject of “fierce debate” during the U.S.-Russia talks but stressed that “the final document … doesn’t mention it” and that the Security Council resolution being negotiated will not be under Chapter 7. He said if Syria fails to cooperate, the Security Council can pass an entirely different resolution “which may employ Chapter 7.”

Lavrov stressed that ongoing attempts to threaten the use of force against Syria would provoke the opposition and disrupt a chance for peace negotiations in Geneva that the U.S. and Russia have been trying to organize.

In London, Syrian Information Minister Omran al-Zoubi said Syria will comply with all Security Council resolutions and will facilitate the mission of the U.N. inspectors in line with the Russian-U.S. agreement. The comments were carried by state-run SANA news agency, which said al-Zoubi made the comments in an interview with Britain-based ITN TV on Sunday.

Meanwhile, invitations were going out Monday to top members of the Syrian National Coalition – the main umbrella opposition group – for an international conference in New York timed to coincide with next week’s U.N. General Assembly meeting, French officials said.

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Associated Press writers John Heilprin in Geneva and Matthew Lee in Paris contributed to this report.

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